The present invention relates to an automatic starting circuit for a split-phase induction motor, and more particularly to an automatic starting circuit for a split-phase induction motor capable of compensating for variable external conditions which may affect the main winding current.
As is well known in the art, motors of the split-phase induction type commonly include at least two windings, a main or running winding used for economical running operation of the motor and at least one start winding used to momentarily assist the main winding in starting the motor. Often, two start winding are employed, one for starting the motor in a forward direction and the second for starting the motor in a direction reverse thereof. In order to insure proper motor operation and, to prevent damage to the start windings, switching means must be provided to automatically energize the start winding whenever the rotor speed is below a predetermined level and to de-energize the start winding when rotor speed exceeds this predetermined level.
The prior art discloses numerous techniques for accomplishing the aforementioned switching operation. For example, centrifugal switches, capable of responding to rotor speed, are well known in the art and have long been used to control the operation of the start windings. An approach of more recent vintage, of which U.S. Pat. No. 3,764,871 is exemplary, involves the use of solid state switching circuitry. This latter approach comprehends the use of means to sense the main winding current and breakover means adapted to energize a bi-directional thyristor connected in series with the start winding whenever the sensed main winding current exceeds a predetermined level. Subsequently, as the rotor speed increases and as the sensed main winding current falls below the break over level of the break over means, the bi-directional thyristor becomes deenergized, thereby preventing current flow through the start winding.
Prior art motor starting circuits, all of which are characterized by a fixed or predetermined threshold level at which start winding switching is accomplished, are inherently incapable of compensating for or responding to external conditions affecting motor operation. Thus, the prior art motor starting circuits are limited to applications where the conditions which affect motor operation are known to vary over a relatively narrow range. Commonly, the prior art starting circuits have required the establishment of a start winding switching threshold (i.e., a predetermined and fixed threshold) which is simultaneously below the minimum expected locked rotor current and above the maximum expected running current. These conditions insure that the start winding will always be initially energized when the main winding current is above the threshold level, and then deenergized as the motor reaches running speed. Although some minimal adaptability to motor sensitive conditions can be achieved by proper establishment of the start winding switching threshold, the prior art starting circuits nevertheless remain unable to compensate for large variations in motor sensitive conditions which frequently affect motor operation.
Conditions which affect the operation of induction motors and, more specifically, induction motor main winding current (thereby influencing the operation of current sensing type motor starting circuits) include variations in the locked rotor and running currents due to the temperature of the motor, variations in the locked rotor and running currents due to differences between individual motors, and variations in the locked rotor and running currents due to variations in the power-line voltage supplied to the motor. For example, a motor designed to operate from a 110 volt a.c. power-line may typically draw a locked rotor current of approximately 13 amperes and a running current of approximately 6 amperes. Under such circumstances, the fixed start-winding switching threshold of the prior art may typically be set at approximately 10 amperes. That is, for main winding currents in excess of 10 amperes the start winding will carry current, whereas, for main winding currents less than 10 amperes the start winding will be deenergized. Since a similar motor operating from a 90 volt a.c. power-line could draw a locked rotor current of only 9 amperes, it will readily be appreciated that, under this 20 volt power-line voltage deviation, the start winding will never be energized and, consequently, the motor will not become operational. Furthermore, if the 20 volt power-line voltage variation occurs in the other direction (i.e. where the a.c. power-line voltage is 130 volts), it will be readily appreciated that the start winding will be energized for an inordinate period of time possibly resulting in damage thereto.
The motor starting circuit of the present invention has, as its key element, the inherent ability to operate successfully over a wider range of varying conditions than the prior art circuits. This includes the ability to operate in cases where the main winding locked rotor current under one set of conditions is less than the main winding running current under another set of conditions.